protection of ships lecture 1: protection against...

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Protection of Ships

Lecture 1: Protection against Corrosion

Lectures 2,3: Protection against Fouling

Colin Anderson

(Business Manager - Antifoulings, International Paint)

Lecture 1: Outline

• Introduction

• Corrosion Processes

• Control of Corrosion

• Introduction to Paint

• Anticorrosion Coatings

• Ballast Tank Coatings

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Corrosion can have disastrous consequences ….


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Financial Times, Wednesday Nov 20, 2002

• “A tanker carrying 70,000 tonnes of industrial fuel split in two and sank off the north-west coast of Spain yesterday.

• The drama surrounding the 26-year old “Prestige” has been deepening since it developed a crack in its hull during a storm last week off Galicia, Spain’s most important fishing region.

• Jacques Chirac, the French president, called for “draconian” maritime security measures to protect Europe’s coastline.”

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Financial Times, Wednesday Nov 20, 2002

• “When a stricken tanker is registered in the Bahamas, owned by a Liberian company, managed by Greek administrators, chartered by a Swiss-based Russian oil trader and sails under the command of a Greek captain with an Asian crew, who is ultimately responsible?

• The sinking has rekindled debate over the inspection of sea-faring vessels and enforcement of international shipping laws …”

• Ballast Tanks are now viewed as a critical area of potential weakness.

“Effective corrosion control in segregated water ballast spaces is

probably the single most important feature, next to the integrity of

the initial design, in determining the ship’s effective life span and

structural reliability.”

- Lloyds Principal Surveyor

Protection of Ships against Corrosion

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• A number of legislative changes has occurred over the last 10 years:

1993 - IACS Unified Requirement UR Z10.2

- Enhanced survey of water ballast tanks introduced for ships inservice

- Coatings classed as being in a good, fair or poor condition

1998 - SOLAS Amendment Ch II-I/Reg 3.2

“Corrosion Prevention of the Sea Water Ballast Tanks”

- Selection, application and maintenance of coatings for water ballasttanks to be approved based on IMO guidelines (Resolution A798)

- Light coloured systems recommended

Protection of Ships against Corrosion: Ballast Tanks

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>20%<20%MinorGeneral Breakdown

>10%<10%MinorHard Scale

>20%<20%

Edges

Weld

>20%

Minor

Minor

Spot Rust

Light Rust

PoorFairGoodRating/Condition

• Enhanced Survey Guidelines


• The enhanced survey programme has possibly had the largest effect

especially as it coincided with the introduction of double hull tankers

• Areas to maintain/inspect have increased by ~ 300%


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• “Erika” was a 25 years old, single

hull tanker, which sank 40 miles

off the Brittany coast, causing

widespread pollution, in 2000.

• IMO was forced to take action,

leading to Marpol 13G - “Post

Erika”.


Post “ Erika “ …….

• “ IACS gets tough on substandard shipping “ - Lloyds List

• Intermediate surveys enhanced to level of special surveys

• Steel thickness measurements to be monitored more closely

• All ships > 20 years to undergo special or intermediate survey if changing

class

Conclusion: Corrosion has become a major issue for ship owners and operators.

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• The design and maintenance of ships requires a knowledge of corrosion:

– Specification and life-time (Fatigue life, use of HTS etc.)

– Age-related defects of single and double hull tankers

– “Survey-friendly” design (ease of access to inspect and repair)

– Quality of build and outfit (fit up of blocks, quality of finishes)

– Tank cargoes (and tank ullage space)


Lecture 1: Outline

• Introduction


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• The driving force behind corrosion of metal objects is the desire of metallic elements to return to the state they are predominantly found in nature e.g oxides.

• Extraction of a pure metal from its ore state requires energy, and it is this energy which makes metals inherently unstable and seek to react with

their environment.

• Metals which have a higher energy input in their production processes are more susceptible to corrosion, and have a lower electrical potential.


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Protection of Ships against Corrosion: Electrical Potentials

High Production Energy Potassium Low Potential

Magnesium

Beryllium

Aluminium

Zinc

Chromium

Iron

Nickel

Tin

Copper

Silver

Platinum

Low Production Energy Gold High Potential

• There are 2 main types of corrosion on ships:

– Atmospheric

– Immersed

• The three essential elements necessary for corrosion to occur are:

– Water

– Contaminants in the water (eg salts)

– Oxygen


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• Corrosion is primarily electrochemical in nature, with a chemical reaction accompanied by the passage of an electrical current.

• In order for this to happen, a difference in electrical potential must exist between different areas of the substrate.

Cathode AnodeElectron Migration

Reduction of ions or oxygen Metal ions eg Fe ++


Cathode AnodeElectron Migration

O2 + 2H2O + 4e �� 4OH- Fe �� Fe++ + 2e-

• The reaction products at the anode and cathode combine to form red iron oxide (= rust):

Fe++ + 2OH - �� Fe (OH)2 , 4Fe (OH)2 + 2H2O �� 4Fe(OH)3

Fe(OH)3 �� FeOOH + H2O , 2FeOOH �� Fe2O3 + H2O


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• The main factors which influence the rate of corrosion are:

– Diffusion

– Temperature

– Conductivity

– Type of ions

– Acidity and Alkalinity

– Electrochemical potential


Factors which control the rate of Corrosion: Diffusion

• Freshly exposed bare steel will corrode at a greater rate than that covered with a compact layer of rust since diffusion of reactants to and from the metal surface is much easier.

• The corrosion rate is heavily controlled by the diffusion of oxygen through the water to the metal surface.

• Areas covered by a thin, conducting, moisture film, such as in emptied

ballast tanks, will corrode faster than areas under immersion.

• The areas at the top of Ballast Tanks and at the top of double bottoms where air has become entrapped, tend to corrode more quickly than deeply submerged areas where oxygen availability is lower.

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Factors which control the rate of Corrosion

Factors which control the rate of Corrosion: Temperature

• Diffusion rates are controlled by temperature, so metals corrode at faster rates at higher temperatures than at lower temperatures.

• Underdeck areas, and regions adjoining the engine room or hot cargo, will

tend to corrode preferentially.

• In modern double hull tankers, with fully segregated ballast tanks, the empty tanks act as insulation from the sea so cargoes retain their heat

longer. So the cargo side of the ballast tank corrodes more quickly than was the case with single hull tankers.

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Factors which control the rate of Corrosion: Conductivity

• For corrosion to occur there must be a conductive medium between the two parts of the corrosion reaction.

• Corrosion will not occur in distilled water, and the rate of corrosion will

increase as the conductivity increases due to the presence of more ions in solution.

• The corrosion rate of steel reaches a maximum close to the normal ionic

content of sea water.

Factors which control the rate of Corrosion: Types of Ions

• Some types of ions present in sea water or in cargoes are more corrosive than others. Chloride ions are usually the most destructive, with sulphateand sulphur containing ions also a major problem.

• Chloride ions destroy the protective properties of any rusts produced, by preventing the formation of the more protective densely packed oxides.

• Sulphur containing ions become involved in additional electron generating

reactions within rust itself. Sulphur can originate from the inert gas system and from cargoes such as crude oil.

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Factors which control the rate of Corrosion: pH

• In sea water (pH ~ 8) the reaction which balances the iron dissolution is the reduction of dissolved oxygen to form hydroxyl ions.

• If the pH falls to 0 (ie becomes acidic) then there is an excess of hydrogen

ions which can become involved in the cathodic reaction to generate hydrogen. Hydrogen and hydrogen ions diffuse very rapidly making the steel corrode more quickly.

• If the pH rises to 14 (ie becomes alkaline) there is an excess of hydroxyl ions and corrosion stops.

• Many blisters found in ballast tanks, particularly double bottoms, are highly alkaline so the steel underneath is very bright.

Factors which control the rate of Corrosion: Potential

• Every metal takes up a specific electrochemical potential when immersed in a conducting liquid. This is called the half potential since it can only be measured by comparing it to another known reference. Common reference electrodes are the Saturated Calomel Electrode (SCE), silver/silver chloride

and copper/copper sulphate.

• The potential that a metal takes up can be changed by connecting it to another dissimilar metal (by using sacrificial anodes) or by applying an external potential (impressed current cathodic protection).

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Other Types of Corrosion: Cavitation

Types of Corrosion: Microbiologically Induced Corrosion (MIC)

• Some of the first double hull tankers showed excessive corrosion in the bottoms of the cargo tanks.

• Black slimes were found, along with a Hydrogen Sulphide smell. These indicated the presence of Sulphate Reducing Bacteria (SRB’s).

• These bacteria have a threshold of activity above ~35C. In single hull tankers the sea water generally cooled the steel to below this temperature, so it was not a problem.

• Corrosion is normally seen as localised pits, filled with a black ferrous product.

• Design of tanks should be such that there are no areas where mud and stagnant water can accumulate.

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Lecture 1: Outline

• Introduction



Corrosion Control

• There are two methods used for corrosion control on ships:

– Modifying the corrosive environment

• Inhibitors

• Cathodic Protection

– Excluding the corrosive environment

• Coatings

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Corrosion Control by Inhibitors

• Corrosion inhibitors are used in areas where the electrolyte solution is of a known and controllable quantity.

• On ships this occurs in onboard equipment (boilers, tanks, pipes).

• Anodic inhibitors work by migrating to the anode and react to form salts

which act as a protective barrier. Examples are chromates, nitrites, phosphates and soluble oils.

• Cathodic inhibitors migrate to the cathode, and either inhibit oxygen

absorption or hydrogen evolution. Examples are salt compounds of magnesium, zinc, nickel or arsenic.

Corrosion Control by Cathodic Protection

• Sir Humphrey Davy and his associate Michael Faraday first suggested this as a method to protect ships hulls in 1824.

• Zinc protector plates were attached to copper sheathed hulls of Navy vessels to reduce copper corrosion. The first full vessel to be protected in this way was “Samarang”.

• The principle of Cathodic Protection is to convert all the anode areas to cathodes, by polarising them to the same electrical potential as the cathodes.

• There are two methods:

– Sacrificial anodes

– Application of an external electric current (ie an impressed current)

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• A lower potential material is placed in electrical contact with the metal surface to be protected.

• The lower potential material becomes the anode and corrodes preferentially.

• Common materials used are Magnesium, Zinc and Aluminium.

Electrical Couple

Lines of +ve currentProtected Structure

Sacrificial Anode

Corrosion Protection by Sacrificial Anode


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Advantages

• No power supply needed

• Simple to instal

• Simple to maintain

• Current cannot be reversed

Disadvantages

• Current depends on anode area - cumbersome on large ships

• Protection only when submerged

• More expensive to maintain

than a DC supply (ICCP)

• Wiring for large anode arrays must be large enough to reduce resistance losses

• An impressed current is used to polarise the anodic areas and balance their electrical potential with that of the cathode.

Original Anode

Original Cathode

C*

Impressed current

AuxilliaryAnode

C* A*

Corrosion Protection by Impressed current

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Cathodic Protection by Impressed current

Corrosion Protection by Impressed current

Advantages

• Current is flexible, to suit any

needs

• Wiring does not need to be large since the voltage can be adjusted to allow for resistance

losses.

Disadvantages

• Continuous DC supply must be maintained

• Current must never be connected in the wrong direction

• Trained personnel are needed

• Current shields are needed if permanent anodes are used

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Liecture 1: Outline

• Introduction




Introduction to Paint

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Introduction to Paint

• Paints are mixtures of many raw materials. The major components are:

– Binder (other terms used include: vehicle, medium, resin, film former,

polymer)

– Pigment or Extender.

– Solvent.

• The first two form the final dry paint film.

• Solvent is only necessary to facilitate application and initial film formation,

it leaves the film by evaporation and can therefore be considered an

expensive waste product.

Binders

• Binders (or “Resins”) are the film forming components of paint.

• They determine the characteristics of the coating, both physical and

chemical.

• Paints are generally named after their binder component, (eg. epoxy

paints, chlorinated rubber paints, alkyd paints etc).

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Liquid Solid polyesterCoal tar resin

Solid epoxy withLiquid curing agent

Binder types

Binder types

• Binders fall into two classes:

- convertible

- non-convertible

• In convertible coatings there is a chemical reaction involved.

• In non-convertible coatings there is no chemical reaction, only loss of

solvent.

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Binders - Convertible

• Stage One: Solvent is lost from the film by evaporation and the film becomes

dry to the touch.

• Stage Two: The film progressively becomes more chemically complex by :

– Reaction with atmospheric oxygen, known as oxidation

– Reaction with an added chemical curing agent.

– Reaction with wate (generally moisture in the atmosphere)

– Artificial heating

– Radiation curing (eg. ultra violet)

Binders - Convertible

Generic types of binders which are in this category include:-

• Oleoresinous varnishes }

• Oil modified alkyd resins } Dry by oxidation

• Urethane oil/alkyd resins } (air drying resins)

• Epoxy ester resins }

• Two component epoxy resins } Dry by chemical

• Two component polyurethane resins } curing

• Moisture cured polyurethane resins } Dry by water

• Silicate resins } absorption

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Binders - Non convertible

• Simple solutions of various resins or polymers are dissolved in suitable

solvent(s).

• Drying is by solvent evaporation – there is no chemical change

• Generic types of binders which are in this category include:-

- Chlorinated Rubber

- Vinyl

- Bituminous

- Cellulose

Note: Antifoulings are a special chemistry, to be discussed in Lects 2 & 3

Pigments and Extenders

• Pigments and extenders are used in the form of fine powders which are dispersed into the binder to various particle sizes.

• These materials can be divided into the following types:-

– Anticorrosive pigments

– Barrier pigments

– Colouring pigments

– Extenders

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Pigments and Extenders - Anticorrosive pigments

• These prevent corrosion of metals by chemical and electrochemical means (chemical inhibition):

– Red lead

– Zinc

– Zinc phosphate

– Zinc chromate

Pigments and Extenders - Barrier pigments

• These decrease permeability of the paint film by making the diffusion path more tortuous.

– Aluminium

– Micaceous iron oxide

Steel

Simplified view of the barrier pigment particles held in the paint film

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Pigments and Extenders - Colouring pigments

• These give permanent colour

– Titanium dioxide - white

– Iron oxide - yellow, red, black

Steel

View of the coloring pigment particles held in the paint film

x1000

Pigments and Extenders - Colouring pigments

Steel

Simplified view of the pigmentparticles held in the paint film

Unrestricted access to the steelPigment not adding to the anticorrosive protection

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Pigments and Extenders - Extending pigments

• These help give the film properties additional properties (eg toughness):

– Crystalline silica

– Bauxite

Steel

View of the extending pigment particles held in the paint film

x1000

Solvents

• Solvents are used in paints principally to facilitate application.

• Their function is to dissolve the binder and reduce the viscosity of the

paint to a level which is suitable for the various methods of application, ie.

brush, roller, conventional spray, airless spray.

• After application the solvent evaporates.

• Solvent therefore is a high cost waste material.

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Solvents

• White spirit

• Xylene

• Butanol

• Iso Propyl Alcohol

• 1,1,1 trichloroethylene

• Acetone

• Methyl ethyl ketone (MEK)

Solvents

View of the Solvent molecules held in the paint film

Solvent

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Pigments(Anticorrosive / Barrier / Colouring / Extending)

Binder (Resin)

Interprime 198

Multiprime

The finished product

Solvent

Convertible (Chemically curing)

Paints

Non Convertible

ChlorinatedRubber paints

Vinylpaints

Bituminouspaints

Coatings Types Summary

MoistureCuring

Oxidativelydrying paints

Two-packpaints

Zinc Silicate Moisture CuringPolyurethane

Polyester Polyurethane Epoxy Alkyd Oil-based Epoxy ester

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Underwater Hull:Anticorrosives &

Antifoulings New buildings and Maintenance:

All area primers, overcoating flexibility etc

Ballast Tanks:

Corrosion Resistance

Decks:Hard wearing

Impact & Chemical

Resistance

Superstructure and Topsides:

High performance cosmetics

Outer hull:Anticorrosives &

Antiabrasionproducts

Cargo & Hold Tanks:

High multi-cargo resistance

Ship Protection by Coatings

KEEL TO RAILANTICORROSIVES- Chlor. Rubber- Pure Epoxy- Modified Epoxy- Coal Tar Epoxy- Vinyl Tar

BALLAST- Pure Epoxy- Modified Epoxy- Coal Tar Epoxy- Vinyl Tar- Pitch Urethane

Not all paint types are suitable for use in permanently or partially immersed areas eg Alkyds

Some paint types are better than others for permanent or partial immersion:Group A/B

ANODE SHIELD- Heavy duty, solvent-

free Epoxies

Group A Products – Chlor. Rubber, Vinyl Tarup to - 1000 mVGroup B Products - Epoxiesup to - 2000 mV

Typical paint types for immersed areas

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Liecture 1: Outline

• Introduction





Underwater anti-corrosion paints

• The outside of a ships hull is generally coated with an anticorrosive paint system and an antifouling paint

• For the purposes of this talk we assume that the antifouling paint has little

or no effect on the anticorrosive properties of the scheme.

• The better the anticorrosive system the longer the vessel will be protected

against corrosion, and the less will be the reliance on cathodic protection.

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Underwater anti-corrosion coatings

There are essentially 4 mechanisms by which a coating can provide protection:

– Physical barrier properties

– Ionic resistance

– Adhesion

– Chemical inhibition

Physical Barrier Properties

• Corrosion cannot proceed more rapidly than the slowest mechanistic step. Therefore limiting the arrival of one of the reactants will slow the overall corrosion process.

• It was assumed for many years that organic coatings protected by acting as a barrier to water and oxygen.

• Systematic study has shown that this may be too simplistic an argument.

Water permeation

• Mayne (1952) showed that the rate of water transport through all paint films studied was at least an order of magnitude greater than that required to support corrosion on unpainted steel i.e. permeation of water through paint films was not rate controlling.

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Water permeation (cont.)

• Work carried out by Haggen and Funke (1975) verified this conclusion:

Water vapour permeability

for 100 mm coating (mg cm-2 d-1 )alkyd resin 2.3

chlorinated rubber 1.0polyurethane 1.4polyester resin 1.3epoxy-coal tar 1.1phenolic resin 1.1nitrocellulose 4.8

Water necessary for corrosion of unpainted steel 0.003 - 0.06 mg cm-2 d-1


Oxygen permeation

• Similar studies also carried out for oxygen permeation through films (Haggen and Funke, 1975). In some cases, oxygen diffusion may be rate controlling :

Oxygen permeability

for 100µµµµm coating (mg cm-2 d-1 )alkyd resin 0.0103chlorinated rubber 0.0022epoxy-polyamide 0.0073VC-VA copolymer 0.0075nitrocellulose 0.106

Oxygen necessary for corrosion of unpainted steel 0.008 - 0.150 mg cm-2 d-1

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Effect of pigmentation

• Barrier action of coatings can be significantly influenced by the presence of inert pigments and fillers with a flake or plate-like shape:

• Polymer structure, cross-link density etc. can also have a profound effect.

Unpigmented:

Short diffusional pathway

Pigmented:

Long diffusional pathway

H20 O2 H20 O2

Coating

Substrate

Ionic Resistance

• In many cases, it is found that the corrosion rate of painted steel is much slower than that allowed by the supply of water and oxygen through thecoating. Some other process must therefore be rate controlling.

• Movement of ions is critical in the charge balance required for the corrosion reactions:

Fe →→→→ Fe2+ + 2 e-

H2O + ½ O2 + 2 e- →→→→ 2 OH-

• Transport of Na+ and Cl- ions to the cathodic and anodic sites is also important.

• Research indicates that where the electrical resistance of a paint is high, good corrosion protection is more likely because the ionic transport is impeded.

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Ionic Resistance

Paint as a resistor

e -

Steel

Fe

Anode

Cathode

Fe 2+

OH-

Na +

Cl -

High resistance of the paint impedes the flow of ions necessary for charge balance.

Paint

As with physical barrier properties, choice of pigmentation will greatly influence the resistance of the paint.

Adhesion

Wet Adhesion

• As discussed previously, the majority of organic coatings have very high rates of water permeation and as such, water cannot be excluded from the interface. The subsequent resistance of a coating to disbondment due to the action of this interfacial water is referred to as “wet adhesion strength”.

• If it is assumed that corrosion processes under the coating have initiated,

then the main benefit of a high wet adhesion value would be in preventing the formation of an interfacial layer of electrolyte connecting the anodic andcathodic sites.

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Adhesion

Na +

Anode

Cl -

e -

Steel

Fe

Cathode

OH-

Paint

Interfacial conducting layer

Fe 2+

Lower resistance along interface “short-circuits” the corrosion cell.

Corrosion enhancement due to poor wet adhesion

Chemical Inhibition

• Chemical inhibition is related to the presence of active anti-corrosive pigments in the binder e.g. red lead, zinc chromates, zinc phosphates (non-

toxic), zinc dust.

• Inhibitive pigments act in the presence of water by leaching out a small fraction from the binder, so making them available at the coating/metal interface. Corrosion is prevented by passivation of the steel or by hindering

formation of ferrous hydroxide.

Zinc Dust

• Initial protection is galvanic. Subsequent protection is due to the formation of insoluble zinc compounds which block pores in the film and render it compact, adherent and impervious (Mayne, 1979).

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Why use paints with Cathodic Protection (CP)?

• CP systems are capable of protecting structures without paint - BUT…...

• CP will only work when the anode and the structure are joined by a conductive medium eg. sea water.

• Structures subject to cyclic wet/dry immersion, eg. splash zones or ballast

tanks, will only be protected for part of the time.

• Classification Societies require a protective coating to be applied in ships ballast tanks.

• The costs involved in the installation and maintenance of a CP system are significantly reduced if a protective coating is applied.

Why use paints with Cathodic Protection?

• All coatings are subject to degradation over their service lifetime

• Coatings become damaged due to the operational environment

• CP systems provide protection at sites of damage or holidays (whilst

the structure is immersed)

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How do CP and paints work synergistically?

• Paint coatings act as a barrier between the structure and the operational environment.

• Transport of detrimental ions such as choride are retarded by paint.

• Coatings reduce the protection current required by placing a resistive path

between the structure and the CP system.


1 2

3

Good Coating / CP Performance

1) Container 27 months ICCP +Epoxy Anticorrosive

2) Container 12 months Zinc Anodes + Vinyl Tar Anticorrosive

3) Bulk Carrier 36 months Anodes +Epoxy (Al) Anticorrosive

NB touch ups - potential weak spots Coating quality is important !

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1) Fair *Anode working, some edge corrosion but coating generally intact

2) Poor *Anode working, edge breakdown, coating delamination

1 2

* American Bureau of Shipping

How do paints and CP work antagonistically?

• ICCP systems can cause paints to blister and lose adhesion if too great a protection current is applied (Anode shields are important )

• Blistering of the coating close to anodes (impressed or sacrificial) can occur, particularly at the stern of ships, where additional CP is required to counteract the galvanic effects of the propeller

• The build up of calcareous deposits (cathodic chalk) at sites of coating damage can lever the paint from the substrate. This causes a ‘jack-up’ effect

• Although the anodic oxidation of iron at the defect is prevented, thecathodic reaction under the coating is accelerated, generating more OH

• Cathodic delamination ….

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Cathodic Delamination

• Reaction with alkali metal ions e.g. Na + ions in sea-water leads to the formation of underfilm alkali (pH = 12-14):

Na + + OH - NaOH (aq)

• The alkali causes chemical damage to the coating, breakdown of barrier properties/ ionic resistance and detachment from the substrate. This process is known as Cathodic Delamination.

• Breakdown of the coating may be continuous or occur as discrete blistering adjacent to the original defect.

• A simliar situation may arise under free-corrosion conditions.

How do paints and CP work antagonistically?

Liecture 1: Outline

• Introduction





• Ballast Tank Coatings

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• Issues effecting coating life (excl surface preparation)

– Variation in dry film thickness

• Too Low �� Corrosion

• Too High �� Cracking and subsequent corrosion

Ballast Tank Coatings

Nº

of R

eadi

ngs

Film thickness

“Idealised” dft distribution

Practical dft distribution

Specified thickness

Average thickness

Typical Application


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• Consequence of low dft at Newbuilding < 1 years service:


• Cracking caused by over-application:


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36 months in service 54 months in service

24 months in service18 months in service

Chemical Tanker - 5 years In ServiceDouble Bottom 301 Port

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OBO - 7 years In Service

• Minor impacts probably as a result of staging / de-staging at newbuilding

• Excellent general corrosion resistance

• Excellent resistance to underfilm creep and blistering

OBO - 7 years In Service

• The mud stains indicate that this tank is frequently used

• Excellent general corrosion resistance

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• Issues effecting coating life (excl surface preparation)

– Variation in dry film thickness

• Too Low �� Corrosion

• Too High �� Cracking and subsequent corrosion

– Thermal Cycling (cargo/ballast/cargo)


Summary

• Coatings are a mixture of binder, pigment, and solvent and help to protect

against corrosion by reducing the diffusion of oxygen and the electrolytic

transport.

• Paints and CP systems work well together providing care is taken with the total

system design.

• Paint choice will influence the cost of corrosion protection at both new

construction and maintenance.

• The direct cost of coatings is ~2% of a new vessel. With the surface

preparation and coating application costs this increases to 8~10%.

• Coating failures can be very costly so choice of the correct product is essential, from new.

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“Castor” 31,068 dwt Product Tanker, built in 1977

• Laden with 29,500 tons of gasoline the vessel encountered severe weather on passage from the Mediterranean, at the end of 2000. A crack extending 22m

across the main deck appeared and forced the master to seek refuge for cargo transhipment operations.

“Castor” 31,068 dwt Product Tanker, built in 1977

• “An exhaustive inspection and analysis of the damaged tanker was carried out by the responsible classification society, ABS, and flag state representative, Cyprus.

• Their preliminary report identified hyper-accelerated corrosion (up to 5mm in 3 years) as the cause of the cracking.

• Three main factors contributed to the rapid deterioration:

– Gasoline was the main cargo, the most corrosive of all oil products

– The critical No. 4 tanks were used for Ballast, introducing salt

– The tanker had been trading in hot regions eg West Africa

• ABS suggests that class rules regarding steel coatings in both cargo and ballast tanks should be re-assessed”

Lloyd’s List Maritime Asia, September 2001.

protection of ships lecture 1: protection against...

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